Research

Photoreceptors and circadian clocks are universal mechanisms for sensing and responding to the light environment. In
addition to regulating daily activities, photoreceptors and circadian clocks are
also involved in the seasonal regulation of processes such as flowering.
Circadian rhythms govern many plant processes, including movements of organs
such as leaves and petals, stomata opening, stem elongation, sensitivity to
light of floral induction, metabolic processes such as respiration and
photosynthesis and expression of a large number of different genes. There is an
intimate relationship between certain photoreceptors and the circadian clock. In
plants, phototransduction not only sets the phase, but also affects the
amplitude and period of circadian rhythms. Members of the
phytochrome family of plant photoreceptors, which can exist in
two photochemically interconversible forms (Pr and
Pfr) and are involved in regulation of plant development and
growth, play important roles in regulating clock activities.

Light affects plant growth and development

Our long term goal is to understand in detail how the phytochrome system
interacts with the circadian clock to regulate the many developmental processes
of higher plants. Our strategy has been to focus on latter steps of the light
signal transduction chain, wherein phytochrome acts in concert with the
circadian clock to alter the expression of specific genes, using examples of
light-regulated genes in Arabidopsis thaliana.

The Lhcb gene family (also designated as CAB genes),
which encodes apoproteins of the light-harvesting complex associated with
photosystem II, has been one of the model systems for studies of both
phytochrome and circadian regulation of gene expression. These studies have led
to the identification of promoter elements involved in phytochrome induction and
circadian rhythm of gene expression. Several proteins interacting with some of
these regions have been identified but until recently, in vivo function
had not been demonstrated for any of them.

We have isolated and characterized a transcription factor, called
CCA1, which binds to a region of an ArabidopsisLhcb
promoter that is necessary for its phytochrome responsiveness ( Wang
et al., 1997 ). CCA1 is a Myb-related protein that binds to at least two of
the Lhcb genes of Arabidopsis, at a sequence that is conserved in
Lhcb genes of many species. Lines of transgenic Arabidopsis plants
expressing antisense RNA for CCA1 showed reduced phytochrome induction of the
endogenous Lhcb1*3 gene. Therefore, CCA1 appears to be a key element in
the functioning of the phytochrome signal transduction pathway leading to
increased transcription of this Lhcb gene in Arabidopsis.

A yeast interaction screen was used to identify a protein that interacts with
CCA1. This protein encodes a novel regulatory (beta) subunit of the protein
kinase CK2 which was designated CKB3. It is the only reported example of a third
beta-subunit of CK2 found in any organism. CK2 beta-subunits stimulate binding
of CCA1 to its binding site on a Lhcb (Cab) gene promoter, and recombinant CK2
is able to phosphorylate CCA1 in vitro.

Figure 1

The formation of a DNA-protein complex containing CCA1 from plant extracts
depends on phosphorylation, and, if CK2 activity in the extracts is inhibited,
the complex cannot be detected in vitro.

Figure 2

The results suggest that CK2 can modulate CCA1 activity both by direct
interaction and by phosphorylation of the CCA1 protein and that CK2 may play a
role in the function of CCA1 in vivo.